This invention relates to the recovery of useful and valuable amounts of iron, and other ferrous alloys, from metal bearing slags and drosses, such as are produced in a steel making plant.
It is well known that in the metallurgical processes used by the steel industry whereby both iron in its usual forms, and various ferrous alloys of greater and lesser complexity are prepared, in addition to the required metals, significant quantities of drosses and slags are also inevitably obtained. Drosses and slags of one form and another are obtained whenever iron, steel, and the various ferrous alloys are handled in a molten state. Thus in addition to being obtained in smelting and alloying processes, these materials are also obtained in casting processes.
The production of these slag and dross materials both cannot be avoided, and represents a potential metal loss to the steel maker, as these materials contain significant amounts of iron, or of ferrous alloys, in the form of trapped metal associated with the oxidic and other materials in the dross or slag, usually derived from furnace slag and fluxing materials used during the handling of the molten metal. For environmental reasons, it is no longer possible for these slag and dross materials simply to be dumped, as has been done in the past. Furthermore, the ever increasing cost of both raw materials and of plant operation, make it necessary for the steel furnace operator, either in smelting to make metal or in a foundry using metal, to minimise metal losses. Although adequate methods exist for dealing with large particle size materials, which are mainly metallic, problems are still encountered in dealing with small particle size materials.
Over the years many and various proposals have been made for dealing with materials of this kind. Generally, most of these methods utilize essentially two steps: the material is comminuted to a smaller size by a dry grinding or crushing step, and then a magnetic separation step is applied to the crushed materials. Where large lumps are concerned, for example Rath, in U.S. Pat. No. 2,971,703, discloses a process in which the screen sizes provide materials ranging from passing 12" inch (approx 300 mm) to passing 1/8th inch (approx 3 mm), the difference in friability of the dross or slag encapsulating the metallic particles appears to be controlling. All of the processes of this type of which Applicants are aware utilize dry crushing or grinding steps.
In certain cases, including silver and copper, it is known that the metal occurs in nature not only in the form of an oxide or sulphide ore, but also as native impure metal intimately associated to a greater or lesser degree with gangue in the form of rock. Lane, in U.S. Pat. No. 644,180 (issued February, 1900), describes a similar approach to recovering the metal from such metal/gangue mixtures. The Lane procedure relies on the fact that these metals are all malleable, and therefore are easily deformed. In essence, Lane proposes to crush the gangue containing native metal in such a way that the gangue is broken up, and the particles of freed metal are flattened out. To achieve a good level of separation, Lane advocates several roller crushing machines be used, in which the rock is sequentially broken down into smaller size pieces. As a consequence largely of the different response of the metal and of the gangue to the crushing forces, the two can be separated by screening. The screens following each roller crusher are sized so that the crushed gangue passes the screen, whilst the flattened out metal particles do not. The feasibility of this process, as Lane points out, depends entirely upon the fact that the native metal being recovered is malleable.
Procedures such as those described by Lane when applied to the finer sizes of ferrous slags and drosses have been found not to be workable. These materials are not amenable to crushing in conventional small size roll crushers. When such materials are fed to the crusher the metal containing particles instead of either being crushed or flattened out, simply jam the rollers. The difficulty is that the ferrous metal present simply is nowhere near malleable enough to respond to such treatment. For such methods to work, the amount of force provided between the rolls in the roll crusher has to be extremely high: this means that the power consumption of such a crusher will be enormous, and the crusher itself has to be built to withstand extreme levels of force. Although the construction of such machinery is theoretically possible, in practice it is easier and more economical to seek alternative ways of achieving the same end, namely to recover at least a major proportion of the metallic values present in these small size materials.
By "small size" in this context is meant ferrous metal containing slag and dross materials typically which will have particles below 12.5 mm in size, with substantial amounts smaller than 1.7 mm, and ranging down to about 0.3 mm.